The term biomagnetometry refers to the detection and measurement of magnetic fields produced by biologic organisms and samples of tissue taken from such organisms. One specialty within the general field of biomagnetometry is magnetoencephalography (often abbreviated by the acronym “MEG”). MEG refers to the detection and measurement of the magnetic fields which are produced by the electric currents which flow naturally within the bodies of humans and animals. For example, such electric current flows are a fundamental feature of the functioning of the neurological system of a human being. Charged ionic flow within the neurons which make up part of the human brain and nervous system is, in effect, an electric current which produces a magnetic field which can be measured using the methods of MEG. The electric currents which drive the pumping of the heart in an animal produce magnetic fields which can be measured using magnetocardiography. Measurements of the magnetic fields produced by these electric currents can be used to deduce information about the size and direction of the currents as a function of time as well as their location and distribution within the body of a person, and therefore to provide information about the state of health and the state of function of the person.
Apparatus and methods of MEG have been developed and expanded over the past forty years, enhancing sensitivity to enable the detection of magnetic fields produced by electric currents flowing deep within the body. The “field of view” of the magnetometers used for MEG has been systematically expanded from single channel detectors of the magnetic field at one location to large helmet-shaped systems measuring the values of the magnetic fields at up to 275 locations around the head of a human being or up to 150 locations over the chest of a human being.
Generally, the biomagnetic measurements of biogenic electric currents are useful for measuring the distribution of such currents in an organ such as a brain or heart. However, one major limitation in the application of the biomagnetic techniques for the purposes outlined above arises from a fundamental property of magnetic fields produced by electric currents flowing in such organs within the body of a human or animal. Any such organs can be described by a circuit of electrically active cells that produce the biogenic current. In intact humans or animals, the tissue of the organ that contains such electrogenic cells is saturated with physiological saline. The physiological saline conducts electricity; thus, the medium containing the saline such as the brain or the head is called a “conductive medium.” The term “conductive” in the context of the current invention refers to the physical property of “electrical conductivity,” and all use of the term “conductive” hereafter will mean “electrically conductive.” From the fundamental principles governing electromagnetism in conductive media, an electric current which flows within and proximate to the surface of such a conductive medium and which flows in a direction which is perpendicular to the surface of that medium produces no net magnetic field external to the medium itself. This is strictly true when the surface boundary between the conducting medium and a non-conducting medium (such as air) is spherical or flat. This factor has limited the utility of biomagnetic measurements such as MEG in providing complete information about electric currents in a tissue or in an organ. The conventional biomagnetic techniques can provide the information only about those components of the electric currents flowing within conducting media which flow in a direction parallel to the surface of that medium, but not the currents which flow normal to the surface. This factor has significantly constrained the application of the biomagnetic techniques for measuring biogenic currents from human and animal brains.
Apparatus and methods for removing this limitation is the field of biomagnetic measurement has been recently developed and has been disclosed in a patent application submitted to the U.S. Patent and Trademark Office on 3 Mar. 2011 and assigned application Ser. No. 13/040,027. This apparatus requires the placement of a non-conducting object known as a primary source mirror (hereinafter PRISM), within a conducting medium in close proximity to the electric currents of interest as they flow within the human or animal body. As noted above, magnetometers for performing MEG are now available which enable the concurrent measurement of the magnetic fields at hundreds of locations on the surface of a human head or elsewhere on the surface of the human body or in a tissue sample for in vitro measurements. However, the limitation of only being able to record the magnetic fields produced by electrical currents flowing in a direction parallel to the surface near the region of interest remains with such large measurement apparatus. This limitation can be removed by use of the basic method of a single primary source mirror as disclosed in the patent application cited above in an expanded manner employing large numbers of such mirrors concurrently.
The use of large arrays of primary source mirrors concurrently for this purpose requires the placement of a conductive medium over the surface of the biological preparation in the region of interest and the placement of such mirrors within this medium and close to the locations of the electrical currents which are of interest. Since these locations are generally not known prior to measurement, it is desirable to place a large number of mirrors immersed in conductive media over the large portion of entire surface of the biological preparation. By doing so, a magnetometer measuring the magnetic fields at hundreds of locations over the surface will measure both the currents flowing parallel to the boundary surface of the conducting medium containing the sample (such as the air-head surface of the head containing the brain) and those flowing perpendicular to such a surface, and thus by vector addition, will measure the magnetic fields coming from electric currents flowing in any direction relative to the surface. Thus, the use of primary source mirrors enables the complete characterization of the biological currents.
The placement of a conductive medium over the boundary surface of the conducting medium (such as the human head or chest) and the immersion of hundreds of primary source mirrors within that medium is a difficult process and one which takes a great deal of time and a great deal of painstaking work to ensure that each mirror is properly oriented and properly fixed in position. There is no known means for performing this task efficiently and effectively if the potential of this method is to be practically realized for all types of biological samples including application to the brain and other organs of a human being or animal, to the neurological system of humans or animals in situ, to tissue samples in a variety of in vitro configurations, and similar types of electrophysiological recording requirements.